This invention is generally directed to submicron conductive composite particles and processes for the preparation thereof, and more specifically the present invention relates to submicron conductive polymeric composite particles, each comprising a polymer and a conductive filler distributed evenly throughout the polymer matrix of composite. The present invention also relates to processes for the preparation of polymeric composite particles. In one embodiment, the process of the present invention comprises the preparation of conductive submicron polymeric particles containing a conductive filler distributed substantially throughout the polymer matrix of the particles, and which particles can be selected as carrier powder coatings. In another embodiment, the process of the present invention comprises the preparation of conductive polymeric composite particles with an average particle size diameter of from between about 0.05 micron to about 1 micron. The conductivity of the generated submicron polymeric composite particles can be modified by, for example, varying the weight percent of conductive filler component present in effective amounts of, for example, from between about 1 weight percent to about 50 weight percent, and also by varying the composition of the conductive filler component. Thus, conductive submicron polymeric composite particles with a conductivity of from between about 10.sup.-10 (ohm-cm).sup.-1 to about 10.sup.-4 (ohm-cm).sup.-1 can be prepared. In one process embodiment, the particles with average diameter of about 0.05 to about 1 micron conductive composite particles are comprised of polymer and a conductive filler distributed evenly throughout the polymer matrix of the composite product, and which product can be obtained by a semisuspension polymerization method as illustrated in U.S. Pat. No. 5,043,404, the disclosure of which is totally incorporated herein by reference. In the aforementioned semisuspension polymerization processes, a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component are bulk polymerized until partial polymerization is accomplished, for example. In one specific embodiment of the present invention, from about 10 to about 50 percent of monomer or comonomers are converted to polymer, thereafter the resulting partially polymerized monomer, or comonomers, is cooled to cease bulk polymerization and to the cooled mixture of polymerized monomer, or comonomers, is added a conductive filler, followed by mixing, using, for example, a high shear mixer until a homogeneous mixer, or organic phase is obtained. Subsequently, the resulting organic phase is dispersed in water containing a stabilizing component with, for example, a high shear mixer, then the resulting suspension is transferred to a reactor and completely polymerized, the content of polymerization reactor is then cooled, followed preferably by washing and drying the polymer product.
Metals such as carrier cores are conductive or semiconductive materials, and the polymeric materials used to coat the surface of metals are usually insulating. Therefore, carrier particles coated completely with polymer or a mixture of polymers can lose their conductivity and become insulating. Although this is desired for some applications, for conductive magnetic brush systems (CMB) the carrier particles should be conductive. Since the carrier polymer coating can be utilized to control carrier tribo, a conductive carrier coating is needed to design carriers with the desired conductivity and triboelectrical properties. Conductive polymers are very costly, and are not suitable for preparing low cost, for example less than $5/pound, coating, thus a conductive polymer composite comprising a low cost polymer and a conductive filler such as conductive carbon black is considered a more suitable alternative.
A polymer composite coating of metal materials, such as carrier beads, is known and can be obtained by two general approaches, solution and powder coating. Solution coating of carriers using a polymer composite solution comprised of a polymer, a conductive filler and solvent can be utilized to prepare conductive carrier, however, trapping of solvent in the solution coating adversely interferes with the use of coated materials, for example the residual solvent trapped in the carrier coating reduces the carrier life, and the release of solvent in the developer housing can cause other problems related to harmful effects of absorbed solvent to various copying machine parts and toxicity of solvent. Moreover, the solvent recovery operation involved in the solution coating processes is costly. The powder coating of metal surfaces can eliminate the need for solvent, and therefore, many of the problems associated with solution coating; however, it requires polymer powder with very small size, for example less than one micron. Although several polymer powders with desired particle size are available for carrier powder coating, submicron polymer composite particles containing conductive filler to prepare conductive coated carriers that maintain their triboelectrical characteristics for extended time periods exceeding, for example, 200,000 images are not known, or available. Therefore, there is a need for conductive submicron polymeric composite particles each containing a conductive filler distributed evenly throughout particles and a process for preparing them.
The preparation of polymeric particles for powder coatings can be accomplished by three methods, namely grinding or attrition, precipitation and in situ particle polymerization. Grinding or attrition, especially fluid energy milling, of large polymeric particles or polymeric composite particles containing fillers to the size needed for powder coating, for example less than one micron, is often not desirable both from an economic and functional viewpoint. These materials are difficult to grind, and therefore, grinding or attrition of required materials for coating with present milling equipment is very costly due to very low processing yield, for example in the range of 5 to 10 weight percent. Precipitation process can also be used to prepare polymeric/polymeric composite particles. In one approach, the polymer solution is heated to above its melting temperature and then cooled to form particles. In another process, the polymer solution is precipitated using a nonsolvent or the polymer solution is spray dried to obtain polymeric/polymeric composite particles. With all these precipitation processes, it has been difficult to achieve low cost, clean (that is, for example, with no or substantially no impurities such as solvents or precipitants in the resulting polymer) particles. It is also difficult to obtain particles with small particle size and narrow particle size distribution. It is also difficult to control filler distribution throughout each particle's polymer matrix. In the in situ particle polymerization process, polymer particles are prepared by using suspension dispersion, emulsion and semisuspension polymerization. Suspension polymerization can be utilized to prepare polymer particles and polymeric composite particles containing, for example, a conductive filler. However, this process does not, for example, enable particles with size less than five microns. Although emulsion and dispersion polymerization can be utilized to prepare polymeric particles of small size, for example less than one micron, these processes wherein particle formation is achieved by nucleation and growth do not enable synthesis of particles containing fillers such as conductive fillers. Conductive fillers, such as carbon blacks, are free radical polymerization inhibitors stopping or at least reducing the rate of polymerization. Moreover, inclusion of fillers to obtain particles with evenly distributed fillers is not believed achievable with the prior art processes mentioned herein. The semisuspension polymerization process is used to prepare submicron polymeric particles each containing conductive fillers which are evenly distributed throughout the polymer matrix.
There is disclosed in U.S. Pat. No. 4,908,665 a developing roller or developer carrier comprised of a core shaft, a rubber layer and a resin coating layer on the surface of the rubber containing conductive fillers for a one component developer. It is indicated in the '665 patent that conductive developing roller can eliminate variation of the image characteristic due to the absorption of moisture for one component development. However, this patent describes a developing roller for one component developer and does not teach the preparation of conductive carrier beads for dry two component developer. U.S. Pat. No. 4,590,141 discloses carrier particles for two component developer coated with a layer of silicon polymer using fluidized bed solution coating. U.S. Pat. No. 4,562,136 discloses as a two component dry type developer which comprises carrier particles coated with a silicon resin containing a monoazo metal complex charging. The two component carriers described in the above two patents are insulating and are not believed to be conductive. There is disclosed in U.S. Pat. No. 4,912,005 a conductive carrier composition coated with a layer of resin containing a conductive particle by solution coating. Residual solvent trapped in the coated layer adversely effects the maintainability of carrier electrical properties for an extended time period.
There is disclosed in U.S. Pat. No. 3,505,434 a process wherein particles for fluidized bed powder coating are prepared by dispersing the polymer in a liquid which is heated to above the polymer melting point and stirred causing the polymer particles to form. The particles are then cooled below their melting point and recovered. However, this process does not, for example, enable particles with a size of below 50 microns.
Also, the suspension polymerization of monomer is known for the formation of polymer/polymeric composite particles generally in a size range of about 200 microns and higher. The main advantage of suspension polymerization is that the product may easily be recovered, therefore, such a process is considered economical. However, it is very difficult by suspension polymerization to prepare very small particles as the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky. For example, there is disclosed in U.S. Pat. No. 3,243,419 a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in the coalescence of the particles. Also disclosed in U.S. Pat. No. 4,071,670 is a method of suspension polymerization wherein the monomer initiator mixture is dispersed in water containing stabilizer by a high shear homogenizer, followed by polymerization of suspended monomer droplets.
Further, disclosed in U.S. Pat. No. 4,835,084 is a method for preparing pigmented particles wherein high concentration of silica powder is used in the aqueous phase to prevent coalescence of the particles. There is also disclosed in U.S. Pat. No. 4,833,060 a process for the preparation of pigmented particles by dissolving polymer in monomer and dispersing in the aqueous phase containing silica powder to prevent coalescence of the particles. However, the silica powder used in both U.S. Patents '084 and '060 should be removed using KOH which is costly, and residual KOH and silica materials left on the surface affects the charging properties of particles. Moreover, the above process does not teach preparation of submicron conductive particles. There is also disclosed in U.S. Pat. No. 3,954,898 a two step polymerization process for the preparation of a thermositting finished powder. However, this process does not enable synthesis of particles with size less than 100 microns. Moreover, this patent does not teach the synthesis of submicron particles containing conductive fillers.
As a result of a patentability search in the aforementioned U.S. Pat. No. 5,043,404, there were located U.S. Pat. No. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; and U.S. Pat. Nos. 4,680,200 and 4,702,988, which illustrate emulsion polymerization. It is known that submicron polymeric particles can be synthesized by emulsion polymerization. However, synthesis of submicron polymeric particles by emulsion polymerization requires a high concentration of emulsifier which remains in the final product and renders it humidity sensitive. Therefore, emulsion polymerization does not enable preparation of clean submicron polymeric particles which are not sensitive to humidity. Moreover, in the emulsion polymerization particle formation is controlled by diffusion of monomer from monomer droplet through a water phase into the growing particles. This mechanism, which is characteristic of emulsion polymerization, does not allow inclusion of conductive fillers in the polymeric particles. Furthermore, it is known that the addition of conductive fillers into emulsion, dispersion or suspension polymerization systems cause severe inhibition which stops or reduces the rate of polymerization significantly.
Disclosed in the aforementioned U.S. Pat. No. 5,043,404, the disclosure of which is totally incorporated herein by reference, is a semisuspension polymerization process for the preparation of small polymeric particles which are comprised of a mixture of monomer or comonomers, a polymerization initiator, a crosslinking component and a chain transfer component which are bulk polymerized until partial polymerization is accomplished. The resulting partially polymerized monomer or comonomers is dispersed in water containing a stabilizer component with, for example, a high shear mixer, then the resulting suspension polymerized, followed by washing and drying the submicron polymeric particles. However, U.S. Pat. No. 5,043,404 does not disclose submicron conductive polymeric particles containing conductive fillers.
There thus remains a need for a submicron conductive polymeric particles, and more specifically conductive submicron polymeric particles containing conductive fillers distributed throughout each particle. Further, there is a need for a process to obtain conductive submicron polymer particles, each containing conductive fillers evenly distributed in the polymer, and more specifically, there is a need for a semisuspension polymerization process for obtaining low cost clean and dry small, for example from between about 0.05 to about 1 micron in average diameter as determined by a scanning electron microscope, polymeric particles containing from about 1 to about 50 weight percent of a conductive filler such as carbon black which is evenly distributed throughout the polymer matrix. Further, there is a need for semisuspension polymerization processes that permit low cost, clean, and dry submicron conductive polymeric particles that can be selected as carrier powder coatings.